In a conventional power semiconductor device, in particular, in a large-current power semiconductor device, its electrode terminal to be connected to an external circuit has to be bonded over a large area in order to cause a large current to flow therethrough efficiently, so that solder bonding has been used for bonding the electrode terminal (see, for example, Patent Document 1). However, as the temperature environment in which the power semiconductor device is used becomes more severe, a possibility arises that the conventional solder bonding may not satisfy the required reliability. Meanwhile, according to the conventional solder bonding, when the power semiconductor device has such a structure in which a ceramic board comprising a ceramic plate and conductor patterns formed on both surfaces of that plate is solder-bonded to a base plate, there is a possibility that the solder which bonds the base plate and the ceramic board together may be remelted by the application of heat at the time the electrode terminal is solder-bonded. For that reason, as the solder for bonding the electrode terminal, it is not allowed to use a solder whose melting point is close to that of the solder which bonds the base plate and the ceramic board together, and thus there is a problem that plural types of solders are required, resulting in complicated process.
As a method of solving these problems, there is a method in which the electrode terminal is ultrasonically bonded to the conductor pattern on the ceramic board. Since ultrasonic bonding is solid-phase bonding without requiring a heating step, it is possible, at the time of bonding the electrode terminal, to bond the electrode terminal over a large area without remelting the solder which bonds the base plate and the ceramic board together, and in addition, it is possible to improve the reliability of the bonding portion in comparison with that by solder bonding. Ultrasonic bonding is a technology for forming a bonding layer by ultrasonically vibrating the materials to be bonded together while applying a pressure to them, using an ultrasonic horn, thereby to remove an oxide film formed along the bonding interface and dirt adhered thereto, and to cause the newly developed surfaces to be tightly adhered to each other. Thus, it is thought that, at the time of bonding, the conductor pattern causes stress concentration and is broken at around the bonding portion of the conductor pattern. In the semiconductor device, at the time of operation, the current that flows in the semiconductor element is flowing in the conductor pattern through the bonding portion from the electrode terminal, and thus, there is a risk that if the conductor pattern is broken, this may result in no current flowing in the semiconductor device. Further, although it is common that the conductor pattern is formed integrally with an insulating layer so as to prevent a current from flowing on the heat dissipation surface of the semiconductor device, if the conductor pattern is broken, the insulating layer formed integrally with that conductor pattern will also be broken at the same time. This causes a risk of not ensuring the insulation property of the product, and is thus very dangerous.
In this respect, for dealing with this problem, in Patent Document 2, such a structure is disclosed in which a resin layer is provided between a terminal end portion and a conductor pattern. Accordingly, in the vicinity of the end of the electrode terminal, a soft member is present between the terminal and the conductor pattern, so that the terminal end portion and the conductor pattern are prevented from making direct contact with each other at the time of ultrasonic bonding, and thus the pressure acting on the conductor pattern is reduced. This makes it possible to restrict the conductor pattern from being broken, so that the bonding can be performed while increasing the applied pressure.
Further, such a method is conceivable in which a condition for bonding is made mild, to thereby prevent breakage of the conductor pattern and the insulating layer. However, if the condition for bonding is made mild, a problem arises that the bonding strength of the bonding portion decreases. For dealing with this problem, in Patent Document 3, such a structure is disclosed in which, as an electrode terminal, a copper member is used that has projections on its end surface to be bonded to the conductor pattern, each having a projection height at least equal to or more than the thickness of the oxide film produced on the surface of the other side, and that is adjusted to have a hardness higher than that of the conductor pattern. Because the projections are provided on the end surface to be bonded of the electrode terminal, the projections will slide on the surface of the other-side member to break/split the oxide film, to thereby make direct contact with the intrinsic surface under that film. In this state, when ultrasonic vibration is further applied continuously, plastic flow occurs in the surface to be bonded of the electrode terminal including portions of the projections. This broadens the mutually adhering portions of the metal intrinsic surfaces, so that the electrode terminal and the other-side member are ultrasonically bonded to each other with a sufficient bonding strength and without being subject to influence by the oxide film.